This invention relates to improvements in an antenna feed system using a corrugated conical horn and operative in a multi-frequency band.
Conventional antenna feed systems of the type referred to have comprised a diplexer connected to a conventional corrugated conical horn for common use with a multiplicity of frequency bands. In such antenna feeding systems, higher modes which are propagable in the respective frequency bands, have been apt to be put in the so-called mode-spike due to the mode resonance between the corrugated conical horn and cutoff points existing in the diplexer. The mode spike has resulted in one of the causes for which the propagation characteristics of the system are distorted. In those antenna feed systems which are operative, for example, in a pair of higher and lower frequency bands, higher order mode spikes in the lower frequency band have been prevented from occurring by selecting the inside diameter of the waveguide section connecting the corrugated conical horn so as to be as small as possible so as to minimize the number of the propagable higher order modes and selecting the depth of corrugated grooves disposed circumferentially on the inner surface of the corrugated conical horn to be less than one half a wave length and greater than or nearly equal to one quarter the wave length in the higher frequency band and also to be small than a wavelength in the lower frequency band so that the inner corrugated surface of the corrugated horn is regarded as the inner smooth surface of conventional horns.
Also, a small number of higher order modes which are propagable in the higher frequency band, have been able to be prevented from being put in the mode spike by properly selecting the dimension and shape of the corrugated grooves.
Those measures have further provided a rotationally symmetrical radiation pattern for the higher frequency band but not resulted in a rotationally symmetrical radiation pattern for the lower frequency band because the depth of the corrugated grooves is too shallow with respect to electromagnetic waves lying in the lower frequency band and therefore, the advantages of the corrugated conical horn have been unable to be effectively utilized.
Accordingly, it is an object of the present invention to provide a new and improved antenna feed system for common use with a multiplicity of frequency bands and exhibiting a rotationally symmetrical radiation pattern over all the frequency bands by means the means of preventing the occurrence of high order mode spikes between a corrugated conical horn involved and cutoff points in a diplexer connected thereto.